A conventional screw press generally has a screw shaft mounted inside an outer screen casing. Slurry is supplied between the screw shaft and the outer screen casing. Slurry is then dehydrated and pressed by rotating the screw shaft subject the slurry to a solid-liquid separation, and the produced sludge is discharged as a cake.
When the cake is formed gradually during the dehydrating operation by the screw press, the load on a drive unit rotating the screw shaft becomes too heavy to press the slurry sufficiently.
The above mentioned outer screen casing mounted on the screen press is not capable of bearing a large pressure. This is because the outer screen casing is mainly formed from a metal screen. The screw press for dehydrating viscous waste water requires a pressure tightness in order to receive a large pressure. Therefore, the metal screen of the outer screen casing mounted on the press is rigidly reinforced by rings, flanges, and so on. The screen of the screw press processing the viscous slurry usually has a fine mesh. As a result, the screen tends to clog and then needs to be cleaned. Conventionally, although the clogged screen is cleaned with a brush, it is very difficult to clean the clogged screen to a good condition because the screen has a very fine mesh and the above mentioned reinforced flange and related structure prevent the brush contacting the entire screen. In an alternative method of cleaning the screen by spraying compressed air onto the screen, that method also is unable to thoroughly remove the clogging.